1. Technical Field
The disclosure relates to apparatus for, and a method of, applying a modulation signal to a phase locked loop, to a phase locked loop comprising the apparatus, and to equipment comprising the apparatus or phase locked loop. The disclosure has application in, particularly but not exclusively, wireless transmitters, for example in mobile communication transceivers.
2. Description of the Related Art
A well-known architecture used in wireless transmitters, for instance for cellular and connectivity applications, applies phase modulation to a local oscillator within a phase-locked loop (PLL). Such an architecture provides not only synthesis, to enable wireless operation on a given channel frequency, but also provides a modulated signal for transmission. Additional paths, for instance I & Q structures to perform Cartesian modulation are not needed. Furthermore a PLL creates precisely the wanted spectrum without the inherent defects of classical modulator circuits, in which mismatch and non-linearity add image, carrier, and inter-modulation spectral components, contributing to EVM (error vector magnitude) and spectral re-growth.
Despite the obvious inherent advantages of applying phase modulation within the PLL, there are a few difficulties. Firstly the loop presents a narrow-band frequency response. Typically the loop cut-off frequency is made sufficiently low so that the local oscillator close-in noise is cleaned-up, while imposing a replica of the crystal reference oscillator spectrum. The integrated phase noise within the PLL loop bandwidth has to remain small, for instance below 1° rms, since this contributes to the overall wireless link performance in terms of residual transmit phase error and a limitation to receiver bit error rate. For a PLL used in a wireless communication system, typical requirements are a loop bandwidth from 10 to 100 kHz, and a close-in noise floor of −90 dBc around the 2 GHz carrier.
Recent radio systems employ wideband modulation schemes in order to achieve high instantaneous data throughput rates. Hence the spectrum of the modulated signal for transmission is usually much wider than the PLL bandwidth. Modulating signals introduced at the reference input, or in the feedback path, from the variable frequency oscillator to the phase comparator, are subject to a low-pass frequency response. The feedback path is of particular interest, since mathematically it can be shown that changing a prescaler divider modulus introduces a precise variation in output frequency, the loop feedback ensuring correct control of the oscillator behavior. This is the first modulation point within a 2-point modulation loop. It has characteristics of precise gain, but restricted pass band. Even if the loop bandwidth could be widened, by accepting a higher overall phase error coming from the integrated noise floor, loop stability dictates that the cut-off frequency should be an order of magnitude lower than the phase comparison reference frequency. This leads to practical PLL bandwidths below one megahertz. Many communication systems use throughputs that occupy a multiple megahertz wide spectrum.
A second modulation point is the local oscillator control input. In an analog PLL this is the voltage control pin that gets its mean potential from the loop filter integration capacitor. Applying an additional voltage signal allows direct frequency modulation of the oscillator. The PLL however attempts to correct for this disturbance. For signals whose frequency falls within the loop bandwidth the loop will create an amplified error signal which will counteract the disturbance and reduce or nullify its effect. For signal frequencies above the loop bandwidth, the loop gain falls-off, reducing the corrective action to zero. This produces a high-pass transfer function, with a transition frequency equal to the loop bandwidth. It has the complementary frequency transfer function characteristic to that of the first modulation point. The gain (change in output frequency for a given voltage input) is set by the intrinsic oscillator gain. This is typically neither well known nor well controlled, being determined by manufacturing of the capacitors used within an LC tank circuit. Furthermore practical circuits show gains that vary with many parameters—supply voltage, temperature, output frequency, as well as part to part dispersion. Therefore there is a need to calibrate the gain in a continuous fashion, during product operation.
As described above, the 2-points can be used in a complementary fashion to apply an arbitrary frequency modulation to the oscillator controlled within a PLL. A problem remains to align the two path gains so as to achieve an ideal flat frequency response. Analysis of various digital modulation schemes shows only a small gain error tolerance in order to attain the required spectral behavior. This is especially the case for complex modulations characterized by a high bit rate per unit bandwidth, such as orthogonal frequency division multiplexing (OFDM) used in wireless LAN (local area network) systems, for example IEEE 802.11a or IEEE 802.11g.